1. Field of the Invention
Embodiments disclosed herein relate generally to subterranean boreholes, and in particular, to systems for controlling the operating pressures within subterranean boreholes.
2. Background
There are many applications in which there is a need to control the back pressure of a fluid flowing in a system. For example, in the drilling of oil wells it is customary to suspend a drill pipe in the wellbore with a bit on the lower end thereof and, as the bit is rotated, to circulate a drilling fluid, such as a drilling mud, down through the interior of the drill string, out through the bit, and up the annulus of the wellbore to the surface. This fluid circulation is maintained for the purpose of removing cuttings from the wellbore, for cooling the bit, and for maintaining hydrostatic pressure in the wellbore to control formation gases and prevent blowouts, and the like. In those cases where the weight of the drilling mud is not sufficient to contain the bottom hole pressure in the well, it becomes necessary to apply additional back pressure on the drilling mud at the surface to compensate for the lack of hydrostatic head and thereby keep the well under control. Thus, in some instances, a back pressure control device is mounted in the return flow line for the drilling fluid.
Back pressure control devices are also necessary for controlling “kicks” in the system caused by the intrusion of salt water or formation gases into the drilling fluid which may lead to a blowout condition. In these situations, sufficient additional back pressure must be imposed on the drilling fluid such that the formation fluid is contained and the well controlled until heavier fluid or mud can be circulated down the drill string and up the annulus to kill the well. It is also desirable to avoid the creation of excessive back pressures which could cause the drill string to stick or cause damage to the formation, the well casing, or the well head equipment.
Referring to FIG. 1, a typical oil or gas well 10 may include a wellbore 12 that has a wellbore casing 16. During operation of the well 10, a drill pipe 18 may be positioned within the wellbore 12. As will be recognized by persons having ordinary skill in the art, the end of the drill pipe 18 may include a drill bit and drilling mud may be injected through drill pipe 18 to cool the drill bit and remove particles drilled by the drill bit. A mud tank 20 containing a supply of drilling mud may be operably coupled to a mud pump 22 for injecting the drilling mud into the drill pipe 18. The annulus 24 between the wellbore casing 16 and the drill pipe 18 may be sealed in a conventional manner using, for example, a rotary seal 26.
In order to control the operating pressures within the well 10 within acceptable ranges, a choke 28 may be operably coupled to the annulus 24 in order to controllably bleed pressurized fluidic materials out of the annulus 24 back into the mud tank 20 to thereby create back pressure within the wellbore 12.
The choke 28, in some well systems, may be manually controlled by a human operator 30 to maintain one or more of the following operating pressures within the well 10 within acceptable ranges: (1) the operating pressure within the annulus 24 between the wellbore casing 16 and the drill pipe 18, commonly referred to as the casing pressure (CSP); (2) the operating pressure within the drill pipe 18, commonly referred to as the drill pipe pressure (DPP); and (3) the operating pressure within the bottom of the wellbore 12, commonly referred to as the bottom hole pressure (BHP). In order to facilitate the manual human control 30 of the CSP, the DPP, and the BHP, sensors, 32a, 32b, and 32c, respectively, may be positioned within the well 10 that provide signals representative of the actual values for CSP, DPP, and/or BHP for display on a conventional display panel 34. Typically, the sensors, 32a and 32b, for sensing the CSP and DPP, respectively, are positioned within the annulus 24 and drill pipe 18, respectively, adjacent to a surface location. The operator 30 may visually observe one or more of the operating pressures, CSP, DPP, and/or BHP, using the display panel 34 and may manually maintain the operating pressures within predetermined acceptable limits by manually adjusting the choke 28. If the CSP, DPP, and/or the BHP are not maintained within acceptable ranges, an underground blowout can occur, thereby potentially damaging the production zones within the subterranean formation 14. The manual operator control 30 of the CSP, DPP, and/or the BHP may be imprecise, unreliable, and unpredictable. As a result, underground blowouts occur, thereby diminishing the commercial value of many oil and gas wells.
Alternatives to manual control may include balanced fluid control and automatic choke control. For example, U.S. Pat. No. 4,355,784 discloses an apparatus and method for controlling back pressure of drilling fluid. A balanced choke device moves in a housing to control the flow and back pressure of the drilling fluid. One end of the choke device is exposed to the pressure of the drilling fluid and its other end is exposed to the pressure of a control fluid.
U.S. Pat. No. 6,253,787 discloses a system and method where the movement of the choke member from a fully closed position to an open position is dampened. An inlet passage and an outlet passage are formed in a housing, and a choke member is movable in the housing to control the flow of fluid from the inlet passage to the outlet passage and to exert a back pressure on the fluid, thus dampening the movement of the choke member. The choke device may operate automatically to maintain a predetermined back pressure on the flowing fluid despite changes in fluid conditions.
U.S. Pat. No. 6,575,244 discloses a system and method to monitor and control the operating pressure within tubular members (drill pipe, casing, etc.). The difference between actual and desired operating pressure is used to control the operation of an automatic choke to controllably bleed pressurized fluidic materials out of the annulus.
During low pressure operations and pump startup, for example, choke systems may encounter mechanical “sticktion.” Sticktion as used herein refers to the temporary adhesion that prevents movement of choke system components, or the slothful reaction in the movement of the choke system components due to the need for an overpressure to initiate movement. This delay may negatively affect borehole operations.
Accordingly, there exists a need for a system capable of tighter control of system pressure (CSP, BHP, and/or DPP) in maintaining the user set point pressure (the desired pressure to be maintained in the casing, drillpipe, or borehole). There also exists a need to improve the operation of choke systems during low pressure operations.